i18n: add english doc

examples/docs/en-US/i18n.md

@@ -56,6 +56,21 @@ Vue.locale('zh-cn', zhLocale)
 Vue.locale('en', enLocale)
 ```
 
+## Compatibility with other i18n plugins
+Element may not be compatible with i18n plugins other than vue-i18n, but you can customize how Element processes i18n.
+
+```javascript
+import Vue from 'vue'
+import Element from 'element-ui'
+import enLocale from 'element-ui/lib/locale/lang/en'
+import zhLocale from 'element-ui/lib/locale/lang/zh-CN'
+
+Vue.use(Element, {
+  i18n: function (path, options) {
+    // ...
+  }
+})
+```
 
 Currently Element ships with the following languages:
 <ul class="language-list">